Complementary metal-oxide semiconductor active pixel sensors (CMOS APS) were originally developed by the Jet Propulsion Laboratory (JPL) for sensor applications that require reduced mass, power, and size in comparison to traditional charge coupled device (CCD) sensors. CMOS APS may be fabricated such that the sensor and the read electronics are present on a single silicon chip. The current CMOS sensor technology utilizes a photo diode with reset, transfer, source follower, and address transistors to implement each pixel. Pixels are often arranged as an array. The pixel array is reset, and then exposed to incoming light for a predetermined integration time. At the end of the integration time, the pixel array is transferred and read through an analog signal processing chain, and the serial digital data is then processed through several digital algorithms to create the final image.
Referring now to FIG. 1, an exemplary CMOS active pixel sensor (APS) pixel 100 is shown. The CMOS APS pixel 100 is reset by turning reset transistor 130 on and transfer transistor 120 on. This causes the voltage across photo diode 110 to be Vt less than supply voltage Vdd 135. Then, reset transistor 130 and transfer transistor 120 are turned off, so that photo diode 110 remains approximately Vt less than supply voltage Vdd 135. Because of charge sharing between transfer transistor 120 and photo diode 110, the final diode voltage after reset will be slightly less than the reset voltage.
In order to read the CMOS APS pixel 100, reset transistor 130 and read transistor 150 are turned on. The reset voltage level is then read using bitline 160. The read transistor 150 and reset transistor 130 are turned off. Transfer gate 140 is then turned on and then off, which transfers any pixel charge stored in photo diode 110 onto floating diffusion 125. Finally, read transistor 150 is turned on, and the pixel level is read. A column decoder uses bitline 160 to present the difference between the reset level and the pixel level to the analog signal processing chain (ASPC).
During the read operation, mismatch on a pixel to pixel level of the reset transistor 130, read transistor 150 and transfer transistor 120 can cause fixed pattern noise in the final image. Also, variations in the column current reference applied to each bit line can cause a column pattern noise in the final image. For this reason, usually, a correlated double sampling (CDS) of the pixels is performed. This CDS is accomplished by reading the reset voltage, reading the pixel voltage, and providing the difference voltage to the analog processing chain, as described above. The CDS approach can significantly reduce the fixed pattern noise in the image.
Another source of noise in the pixel is the charge transfer noise. At the end of the integration time, the charge in photo diode 110 is transferred to the floating diffusion 125 between the reset transistor 130 and the transfer transistor 120.
The goal is to transfer all of the charge from the photo diode 110 to the floating diffusion 125. Since the transfer is dependent on the photo diode voltage at the end of integration, the physical design of the photo diode 110, transfer gate 120, and floating diffusion 125 are very important.
In an APS system, the photo diode is allowed to accumulate charge during the integration time, and at the end of the integration time, the photo diode voltage is read. This voltage is then transformed into a digital signal that represents the incident power of light at the pixel. The relationship between power, time and voltage is given by       P    I    =                    V        PD            ·              C        PD            ·      h      ·      c              q      ·              Q        E            ·              T        I            ·      λ      where                PI is the incident light power,        VPD is the photo diode voltage,        CPD is the photo diode capacitance,        QE is the quantum efficiency,        TI is the integration time,        h is Planck's constant,        c is the speed of light,        q is the electron charge, and        λ is the wavelength.        
At the end of the integration time, the power levels can be grouped into two categories, the saturated powers, and the unsaturated powers. Referring now to
FIG. 2, a plot 200 of APS photo diode voltage versus integration time is shown. Saturated powers P1 and P2210 are mapped into the maximum level of the analog-to-digital conversion (ADC), while unsaturated powers P3 and P4220 can be mapped into distinct levels. Also, the saturated powers can cause blooming in the pixel array. Once a pixel has reached its saturation level, the charge generated in the photo diode of this pixel escapes to other pixels in close proximity. This causes the other pixels to accumulate charge not associated with their incident light, and consequently the other pixels are read out as a higher intensity level.
Referring now to FIG. 3, a side view of a photo diode under high intensity illumination 300 is shown. Photo diode 310 is receiving high intensity illumination 340, while a neighboring photodiode 320 is receiving low intensity illumination 350. High intensity illumination 340 causes photo diode 310 to drop below ground, which then turns on parasitic transistor 330. Blooming occurs when high intensity light causes a photo diode to drop below ground, which turns on the parasitic transistor 330. When parasitic transistor 330 is on, its collector induces current that artificially drops the photo diode 320. This causes the pixel associated with photo diode 320 to detect a higher illumination than the low intensity illumination 350. The current of parasitic transistor 330 causes a blooming effect, in which the image intensity is overestimated in regions surrounding a high intensity illumination.
There are many processing variations that have been used to try to capture the excess charge before it reaches an adjacent pixel, such as deep implants, trenches, etc. However, for the active pixel sensor, only unsaturated powers can be detected. There is therefore an unmet need in the art for an active pixel sensor that is able to isolate the effects of blooming, and to enable power levels occurring under saturation conditions to be detected.